1. Field of the Invention
The present invention relates to a calibration verification device which eliminates the need for calibration gases to verify the calibration of a capnometer and any similar devices for measuring gas concentrations. This is accomplished by simulating the infrared absorption of calibration gases using thin films which are mechanically placed in front of the infrared sensor during calibration.
2. Brief Description of the Prior Art
Capnometers function by passing light of a specific wavelength (typically infrared) through a gas and measuring the amount of light that is absorbed. Typically, capnometers measure the amount of carbon dioxide in a patient's breath for medical diagnostic purposes. For this purpose, conventional capnometers comprise an airway adapter, a sensor and a processor box. The airway adapter is typically a hollow aluminum casting with sapphire windows which is inserted in series with the ventilator plumbing and is used to keep the patient's respiratory gases from coming in contact with the sensor mechanism. However, disposable plastic airway adapters are known, as described in U.S. Pat. No. 5,067,492. The sensor is snapped over the airway adapter windows, and the measurement is made directly on the artificial airway through which the patient is breathing. The sensor contains all the optical components necessary to make the infrared measurement and is connected to the processor box by a cable. The processor box powers the sensor, processes the return signal, and presents the data via an LED display.
In order to ensure accurate measurements, capnometer sensors are typically calibrated by taking measurements of sample gases of known concentration prior to taking on-line measurements. For example, in the system of U.S. Pat. No. 5,206,511, gas compules containing gases of known concentration are used for calibrating the sensors. Unfortunately, such gas compules may leak, which, of course, affects the accuracy of the calibration. Also, using sample gases for calibration is cumbersome since it is necessary to keep track of the sample gases and assure the accuracy of the concentrations of the sample gases.
It is desired to develop a calibration verification device which eliminates the need for sample gases to verify the calibration of a capnometer. It is also desired that such a calibration verification device include a simple mechanism for actuation so that the calibration verification process is simple and short in duration. The present invention has been designed to meet these needs.